The field of the present invention is that of hydraulic elevators. More particularly, the field of the present invention is that of a hydraulic elevator with a brake and a method of utilization thereof.
Elevators come in two main types. One type is a cable elevator wherein an electric motor is rotatively connected with a drum. The drum has a traction cable wrapped over the drum. One end of the cable is attached to an elevator car. The other end of the cable is attached to a counterweight. The elevator car and counterweight are moved up and down opposite one another by rotation of the drum. A cable elevator requires a structural support which can hold the electric motor and drum on top of a building structure. The physical structure required for an elevator shaft and for the support of the elevator motor and drum makes a significant contribution to the cost of a cable elevator. In building structures of six stories or less, it is common to provide a hydraulically actuated elevator. A hydraulic elevator does not require as much structural support of the elevator shaft as a cable elevator requires. Also the hydraulic elevator does not have an overhead motor. In the common type of hydraulic elevator, a powering cylinder is positioned at a subterranean level. Slidably and sealably mounted within the cylinder is a piston often referred to as a plunger. The plunger is sealed by ajack head which is mounted on top of the cylinder. The plunger is made from a hollow piece of steel which has an interior which has been sealed off. The plunger has an exterior wall which is highly polished. To move the elevator car, pressurized fluid is pumped into the cylinder to extend the plunger upwards. To lower the elevator car, pressurized fluid is released from the cylinder. The cylinder must be at least as long as the amount of extension desired for the plunger. In some applications, the length of extension can approach 60 feet. Since many buildings have a basement, the cylinder can sometimes extend 60 feet below an elevator pit which is below a basement floor.
As mentioned previously, in most applications the cylinder extends below the basement floor of the building. Leakage of the cylinder is therefore very hard to detect except by closely monitoring the fluid level in the reservoir of the hydraulic pump which supplies hydraulic oil to the cylinder. The cylinder is subject to pressures in the neighborhood of 50 to 500 pounds force per square inch (psi). A catastrophic failure of a bottom plate of the cylinder can sometimes allow the plunger to descend at a faster rate than desired. To guard against catastrophic failures of the bottom plate, a double bottom cylinder design became the industry standard in 1971. The double bottom cylinder design features a bottom plate plus a bulk head equipped with a relief orifice. The orifice limits the speed of the plunger's descent should a bottom plate catastrophic rupture occur. The double bottom cylinder design is a major improvement. However, many hydraulic cylinders were installed in service before the double bottom cylinder became the industry standard. Therefore, it is desirable to provide a means of restraining downward plunger movement in cases where there is a catastrophic failure of a single bottom cylinder.
Various brakes for the plunger have been brought forth. However, certain technical limitations of prior plunger brakes have discouraged their utilization. Many of the prior art plunger brakes have provided cams or shoes which have been positioned by levers. Actuation of the brake causes the levers to pivot and bring the brake cams or shoes into engagement with the plunger and come to an over center position forcing the brake cams or shoes into the plunger and therefore restraining its motion. As mentioned previously, the plunger is a hollow piece of steel. Often prior plunger brakes inadvertently cause deformation of the plunger. Deformation of the plunger or gouging of its polished cylindrical surface causes major sealing problems. Another problem of many of the prior art plunger brakes is their physical height. The plunger brake must fit between a gland ring (which is on a top end of the jack head) and the bottom of the elevator car when the elevator car is in its lowermost position. In a new installation, the cylinder and jack head can be lowered to provide more room for the plunger brake. However, in attempting to retrofit older hydraulic elevator installations, the option of lowering the hydraulic cylinder and its jack head to make space is not available since the building foundation would have to be torn up and a hole would have to be excavated below the cylinder to allow it to be lowered. Such an effort is often cost prohibitive.